The use of heat exchangers for cooling a range of electronic devices is known in the art. Liquid cooled heat exchangers are generally characterized as having macro-channels, mini-channels, or micro-channels, depending on the size of the channels. The term ‘micro’ is applied to devices having the smallest hydraulic diameters, generally between ten to several hundred micrometers, while ‘mini’ refers to diameters on the order of one to a few millimeters, and ‘macro’ channels are the largest in size, generally greater than a few millimeters. An example of a typical macro channel design is the conventional swaged-tube cold plate illustrated in FIG. 1a. 
As shown in FIG. 1a, the prior art swaged-tube cold plate includes a copper tube 11 swaged into grooves machined in an aluminum plate 13. Swaged tubes are generally suitable for cooling large-area devices, particularly when cost is a factor, and/or when the cooling requirements do not require a very low thermal resistance. The lowest thermal resistance that can generally be achieved with a conventional swaged-tube cold plate is approximately 2° C./(W/cm2). Because of these limitations, applications requiring lower thermal resistances often use finned cold plates, such as the prior art finned cold plate shown in FIG. 1b. 
Conventional finned cold plates have a number closely spaced fins 21 attached to the heat transfer surface 23. The fluid flows through the channels 25 formed by the spaces between the fins. The channels typically have a width between about 1 to 5 mm. Conventional finned cold plates can achieve thermal resistances as low as approximately 1° C./(W/cm2).
The thermal resistance of macro channel cold plates decreases as the flow rate is increased and approaches asymptotically a minimum value at a flow of about 0.1 LPM/cm2. Increasing the flow rate further has not been found to result in an additional reduction in the thermal resistance.
For cooling high heat flux devices, such as solid-state laser diodes, which dissipate heat at a rate of 500-1000 W/cm2, cold plates with substantially lower thermal resistance than that of the swaged-tube cold plates or the machined fin cold plates are needed. In these applications, micro-channel cold plates are generally employed.
There are two primary types of prior art micro-channel cold plates: parallel flow and normal flow. As the name implies, parallel flow micro channel cold plates have the liquid flowing through the heat transfer passages in a direction parallel to the surface being cooled. In contrast, normal flow micro channel cold plates (NCP) have the liquid flowing through the heat transfer passages in direction normal to the surface being cooled. The parallel flow cold plates have geometries similar to that of the finned cold plate shown in FIG. 1b, except that the dimensions are scaled down by an order of magnitude. For example, the channel width in a micro-channel cold plate is typically less than 500 microns. Because of the high pressure drop in the micro-channels, the size of the parallel flow micro-channel cold plates is typically less than about 1 to 2 cm on a side. Even at these small sizes, the pressure drop can be too large for some applications. The pressure drop can be reduced by subdividing the micro-channel into several sections and providing alternating inlet and outlet manifolds along the length of the cold plate, for example as described in U.S. Pat. No. 6,986,382 to Upadhya.
One objective in the design of micro-channel cold plates is to minimize the pressure drop consistent with achieving the target thermal performance. Minimizing the flow length and maximizing the flow area of the micro-channels is most often employed to achieve this objective. Conventionally, the flow length is minimized by making the flow axis straight, while the flow area is maximized by making the micro-channel depth large compared to its width. As such, prior art parallel-flow micro-channels have a depth that is an order of magnitude larger than the width.
Normal flow cold plates invented by the present inventor, Javier Valenzuela and as described in U.S. Pat. Nos. 5,145,001 and 6,935,411 among other patents, demonstrate excellent heat transfer effectiveness, especially in high heat-flux applications. However, for some systems the highly effective cooling provided by the normal flow design is not required, and the cost of the heat exchanger may not be warranted. FIG. 2 shows one example of a cross-section of a prior-art normal flow micro-channel cold plate. Normal-flow micro-channel cold plates incorporate a low-pressure drop manifold structure 27 that distributes and collects the flow over the active area of the cold plate. The micro-channels 29 are embedded in a thin layer between the manifold structure and the active surface 31. The micro-channels direct the fluid in a direction substantially normal to the active surface: first from the manifold structure towards the active surface, and then from the active surface towards the manifold structure. The total length of the micro-channels is very short, about twice the thickness of the heat transfer layer and, therefore, the pressure drop in the normal flow micro-channel cold plates is small, even at the high flow rates per unit area required in these high heat flux applications.
In spite of the order of magnitude lower thermal resistances that can be obtained through the use of micro-channels, they are seldom used in large-area cold plates. The principal objections to the use of micro-channels in large area cold plates are: (1) the large pressure drop associated with the flow through long, small-hydraulic-diameter passages, and (2) the relatively high cost of fabricating passages with such small dimensions.
There are also other methods of cooling that utilize fluid flowing through channels in order to cool a device. For example, U.S. Pat. No. 6,213,194 discloses the use of a hybrid cooling system for an electronic module which includes refrigeration cooled cold plate and an auxiliary air cooled heat sink. The '194 patent also discloses the use two independent fluid passages embedded in the same cold plate to provide redundancy. A single serpentine passage, akin to that of a swaged tube cold plate, or multiple straight passages feed by headers, akin to a finned cold plate is used for each one of the redundant systems.